When a vehicle is traveling on a road surface having a low friction coefficient, such as a frozen road, a wet paved road, or a dry paved road where sand is disposed, the grip of the tires with respect to the road surface falls, so that the wheels tend to slip. Thus, heretofore, with a brake system for braking the rotation of wheels mounted on the vehicle (first brake), even if hard braking is activated, the braking distance may increase excessively so that the vehicle may collide against an obstacle, irrespective of whether the vehicle is traveling at a high speed or a low speed.
On a road surface having such a low friction coefficient, since the degrees of slipping tend to be uneven for individual wheels, travel of the vehicle tends to be unstable, so that the vehicle may collide against an obstacle in an oblique direction. As means for allowing stable travel of a vehicle, such systems as an ABS (Antilock Brake System), which detects the degree of slip for every wheel and controls the braking force of each wheel, and VSC (Vehicle Stability Control) for controlling the engine output and the braking force of the wheels by detecting the steering tendency of the vehicle with a yaw rate sensor and a lateral acceleration sensor are used practically. But in order for these systems to operate effectively, it is a prerequisite that the grip of the tires remain to a certain extent.
In order to avoid collision of a vehicle on a road surface having a low friction coefficient as mentioned above, apart from a conventional brake system, it is proposed to mount a second brake for braking the vehicle in a short distance by increasing the frictional resistance with the road surface. As the second brake, there are ones which spray slip-preventive materials such as sand and ice particles between the tires and the road surface (See for example patent documents 1 through 3.), ones in which a liquid adhesive is applied to the tire surfaces to stick slip preventive materials to the tires (See for example patent document 4.), ones in which thawing of the frozen road is prevented by blowing cold air on the tires (See for example patent document 5.), ones in which a braking plate or a toothed braking wheel is pressed against the road surface (See for example patent documents 6 through 8.), and ones in which the pressing force of the vehicle against the road surface is increased by inflating a gas bag outwardly of the vehicle (See for example patent document 9.), etc. (patent document 1; unexamined JP patent publication 4-38204, patent document 2; unexamined JP patent publication 7-309101, patent document 3; unexamined JP patent publication 8-25905, patent document 4; unexamined JP patent publication 63-2706, patent document 5; unexamined JP patent publication 50-100703, patent document 6; unexamined JP patent publication 49-2228, patent document 7; unexamined JP patent publication 54-122528, patent document 8; unexamined JP patent publication 8-40222, patent document 9 unexamined JP patent publication 6-286586.)